(Metal/rubber) composites for tires are known and have been described in a very large number of documents. They are generally constituted of a rubber matrix reinforced with reinforcing members, most frequently in the form of wires or assemblies of wires, of perlitic (or ferro-perlitic) carbon steel, hereinafter referred to as “carbon steel”, the carbon content of which is normally between 0.35% and 1.2% (% by weight).
It is known that these composites, which are subject to very great stresses during running of the tires, in particular to repeated flexing or variations in curvature, must satisfy a large number of technical criteria, which are sometimes contradictory, such as uniformity, flexibility, endurance under bending and in compression, tensile strength, resistance to wear and to corrosion, and keep these performances at a very high level for as long as possible. It will readily be understood that the adhesive interphase between rubber and metal plays a major part in the durability of these types of performance.
The traditional process for joining the rubber compositions to the carbon steel consists of coating the surface of the carbon steel with brass (copper-zinc alloy), the bond between the carbon steel and the rubber composition being provided by sulphurisation of the brass during vulcanization. To ensure optimum adhesion, furthermore frequently an adhesion-promoting additive such as a cobalt salt is used in the rubber composition.
The brass coating has the known disadvantage that the adhesion between the carbon steel and the rubber matrix is liable to weaken over time, owing to the gradual evolution of the sulphides under the action of the various stresses encountered, in particular thermal and/or mechanical stresses. Furthermore, the presence of humidity in the tires plays a very important part by accelerating the above degradation process. Finally, the use of a cobalt salt makes the rubber compositions more susceptible to oxidation and significantly increases the cost thereof.
Admittedly, here or there wires or cables of carbon steel have been proposed which have different alternating coatings, in particular alloys of zinc such as zinc-cobalt or zinc-aluminum (see for example Wire Journal International 31, No. 10, October 1998, pp 78-82; WO91/01389 or equivalent patents EP-B1-0 483 198; U.S. Pat. No. 5,342,699), but until now without real success or commercial development.
In particular, application WO91/01389 proposes, as a replacement for the brass, a specific zinc-aluminum alloy, having a low percentage of aluminum, which has been known for a long time for its corrosion resistance, this alloy possibly being covered by a second layer of metal or metal alloy, in particular of cobalt, to improve the adhesion. Although the wires and cables described have improved corrosion resistance, the levels of adhesion achieved are insufficient, less than those offered by a conventional brass coating.
Thus, despite the aforementioned drawbacks of brass, the latter today still constitutes the reference adhesive interphase, the one which is by very far most used in (carbon steel/rubber) composites for tires, in particular in carcass or crown reinforcements therefor.